delayed-mode-manual by ashrafp

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									Argo Delayed-Mode Manual

Last updated: 28 Oct 2003



Table of Contents

1. Overview
2. Delayed-mode procedures for salinity
       2.1. Quality control and statistical test
       2.2. Assigning delayed-mode salinity adjustments, error estimates, and qc flags
       2.3. What to say in the scientific calibration section of the netcdf file
       2.4. Timeframe for availability of delayed-mode salinity data
       2.5. Summary flowchart
3. Delayed-mode procedures for pressure
4. Delayed-mode procedures for temperature




WARNING: THIS DOCUMENT IS A DRAFT ONLY.
To be discussed at the ADT Meeting at Monterey.
Please send comments to Annie.Wong@noaa.gov
1. Overview
        The Argo real-time system provides primary qc to all float measurements and
delivers them to users within the 24-48 hrs timeframe. Additional quality control is then
applied to float measurements in delayed-mode. This document describes Argo delayed-
mode procedures applicable to various parameters measured by the floats.



2. Delayed-mode procedures for salinity
        The free-moving nature of profiling floats means that most float salinity
measurements are without accompanying in situ “ground truth” values for absolute
calibration (such as those afforded by shipboard CTD measurements). Therefore Argo
delayed-mode procedures for salinity rely on statistical methods for detecting artificial
trends in float salinity measurements. However, since the ocean has inherent spatial and
temporal variabilities, Argo delayed-mode salinity qc is accurate only to within the
associated statistical uncertainties. Users therefore should include the supplied error
estimates in their usage of Argo delayed-mode salinity data.

2.1. Quality control and statistical test

        The real-time qc procedures issue a set of qc flags that warns users of the quality of
the float salinity. These are found in the variable PSAL_QC. Float salinity with PSAL_QC
= 4 are unadjustable and so are not considered in delayed-mode.

       In delayed-mode, float salinity that have PSAL_QC = 1, 2, 3 are further evaluated.
Anomalies in the relative vertical salinity profile, such as measurement spikes, that are not
detected in real-time, are identified. Float salinity with PSAL_QC = 1, 2, 3 that fail this
evaluation are considered unadjustable.

        Float salinity with PSAL_QC = 1, 2, 3 that pass this evaluation are considered
adjustable in delayed-mode. Adjustable float salinity are assembled into time series, or
float series. Sufficiently long float series are compared with statistical recommendations
and associated uncertainties. These statistical recommendations are obtained by accepted
methods, in conjunction with appropriate reference database, and require good/adjusted
float pressure and temperature. Please refer to Section 2 for Pressure and Section 3 for
Temperature.

       Results of this series comparison will fall into two categories:

(A). difference in salinity, ΔS, between float series and statistical recommendations, is less
than 1 x statistical uncertainty;
(B). difference in salinity, ΔS, between float series and statistical recommendations, is
greater than 1 x statistical uncertainty.
2.2. Assigning delayed-mode salinity adjustments, error estimates, and qc flags

        The PI (here, PI means Principal Investigator or responsible persons assigned by the
PI) then evaluates results from the series comparison and assigns salinity adjustments, error
estimates, and qc flags. In Argo netcdf files, these are found respectively in the parameters
PSAL_ADJUSTED, PSAL_ADJUSTED_ERROR, and PSAL_ADJUSTED_QC. These
three parameters are to be considered together and not separately. The original float salinity
and related qc flags will remain in the parameters PSAL and PSAL_QC, and are never
altered in delayed-mode.

     The following is a set of guidelines for assigning values to PSAL_ADJUSTED,
PSAL_ADJUSTED_ERROR and PSAL_ADJUSTED_QC in Argo netcdf files.

For unadjustable float salinity

PSAL_ADJUSTED = PSAL;
PSAL_ADJUSTED_ERROR = FillValue;
PSAL_ADJUSTED_QC = 4.

For adjustable float salinity in Category (A)

i). If the PI accepts float salinity as good,

PSAL_ADJUSTED = PSAL;
PSAL_ADJUSTED_ERROR = uncertainty from statistical analysis;
PSAL_ADJUSTED_QC = 1.

ii). If the PI does not accept float salinity as good and can provide alternative correction,

PSAL_ADJUSTED = correction provided by PI;
PSAL_ADJUSTED_ERROR = correction uncertainty provided by PI;
PSAL_ADJUSTED_QC = 1.

iii). If the PI does not accept float salinity as good but cannot provide alternative correction,

PSAL_ADJUSTED = PSAL;
PSAL_ADJUSTED_ERROR = uncertainty from statistical analysis + |ΔS|;
PSAL_ADJUSTED_QC = 2 or 3.

For adjustable float salinity in Category (B)

i). If the PI accepts statistical recommendation,

PSAL_ADJUSTED = value recommended by statistical analysis;
PSAL_ADJUSTED_ERROR = uncertainty from statistical analysis;
PSAL_ADJUSTED_QC = 1.
ii). If the PI does not accept statistical recommendation and can provide alternative
correction,

PSAL_ADJUSTED = correction provided by PI;
PSAL_ADJUSTED_ERROR = correction uncertainty provided by PI;
PSAL_ADJUSTED_QC = 1.

iii). If the PI does not accept statistical recommendation but cannot provide alternative
correction,

PSAL_ADJUSTED = PSAL;
PSAL_ADJUSTED_ERROR = uncertainty from statistical analysis + |ΔS|;
PSAL_ADJUSTED_QC = 2 or 3.



2.3. What to say in the scientific calibration section of the netcdf file

        Within each single-profile Argo netcdf file is a scientific calibration section that
records scientific details of delayed-mode adjustments. In this scientific calibration section,
for every parameter listed in PARAMETER (and here we are concerned only with PSAL),
there are three fields to record scientific calibration details:

- SCIENTIFIC_CALIB_EQUATION;
- SCIENTIFIC_CALIB_COEFFICIENT;
- SCIENTIFIC_CALIB_COMMENT.

        In cases where no adjustment has been made (ie. where PSAL_ADJUSTED =
PSAL), SCIENTIFIC_CALIB_EQUATION and SCIENTIFIC_CALIB_COEFFICIENT
shall be filled by their respective FillValue. SCIENTIFIC_CALIB_COMMENT shall
contain wordings that describe the evaluation, e.g. “No adjustment is needed.”

         In cases where adjustment has been made, examples of wordings can be:
SCIENTIFIC_CALIB_EQUATION: “C^ = r * C, where C is float potential conductivity
referenced to 0 dbar. C is a derived quantity and is calculated from float salinity. The
adjusted C^ is converted back to give adjusted float salinity.”
SCIENTIFIC_CALIB_COEFFICIENT: “r = 1.000265330617906”
SCIENTIFIC_CALIB_COMMENT: “Sensor drift detected. Adjusted float salinity to
statistical recommendation as in WJO (2003).”

       Basically, the PI is free to use any wording he/she prefers. Just try to be precise and
informative.
2.4. Timeframe for availability of delayed-mode salinity data

        The statistical methods employed by the Argo delayed-mode process for salinity
require the accumulation of a float series for reliable evaluation of the sensor trend.
Timeframe for availability of delayed-mode salinity data is therefore dependent on the
sensor trend. Some floats need a longer float series than others for stable calibration. Thus
delayed-mode salinity data for the most recent profile may not be available until sufficient
subsequent profiles have been accumulated. From experience, the timeframe is usually
between 6 to 12 months after a profile is sampled.



2.5. Summary flowchart

								
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